SUCCESSION AND REPLACEMENT IN DEVELOPMENT OF SILURIAN BRACHIOPOD POPULATIONS

Fluctuating water depths in platform seas covering central North America during late Llandovery time introduced shifts in the spectrum of coral-algal, pentameran and stricklandian communities. Opportunistic orthotetacean brachiopods were common during these times of community replacement. A pavement constructed by their flat shells was sometimes used as the initial substrate for attachment by the spat of pentameran brachiopods. Both orthotetacean and pentameran shells served as the initial substrate for stricklandian brachiopods. With the decline of a pentameran or stricklandian population, disarticulated shells provided a substrate for repopulation. An assemblage of immature Pentamerus in life position indicates that the typical umbo-down posture was an early result of maximum packing. Development of more advantageously placed individuals severely restricted the growth of others trapped between the substrate and the beak regions of surrounding neighbors. After pentameran assemblages were buried in life position, scouring of the sea bottom frequently produced erosion surfaces which truncated their thin shells. These surfaces were sometimes penetrated by boring organisms, and were firm enough for brachiopod repopulation.